Some wireless networks are centrally controlled, with a single device coordinating the operation of all of the devices in the network. This central coordinator provides instructions to each of the devices regarding transmission times, power levels, and the like. One example of this sort of network is a piconet using the proposed IEEE 802.15.3 standard.
In such a centrally-controlled network, all of the devices in the network must be able to both send and receive data from the central coordinator. Any device that cannot reliably communicate with the central coordinator of a network cannot join the network.
This means that a local device that can physically communicate reliably with a target device (i.e., it is within operational range of the target device) might not be able to effectively communicate with that target device if the target device is already in a network. In such a case, the local device would have to also be able to communicate with the central coordinator to be assigned the necessary channel time for such a communication.
If the local device can't communicate with the central coordinator, then it won't be able to join the same network that the target device is in to ask to speak to that device. And since the coordinator allocates resources for communication in the network, if the local device is not in the same network as the target device, then the target device will never be assigned any time to communicate with the local device.
In some protocols the target device could create a child network within the main network it belongs to facilitate communication with the local device. In this case the target device would take on the properties of the central coordinator with respect to that child network, and its limits of communication would set the boundaries of the child network. Once the child network was started, the local device could become a member of that child network and request communication with the target device.
However, with current networks, there is no way for the target device to know that there is another device looking to communicate with it. And absent the knowledge that there was another device that would benefit from the creation of a child network, the target device has no reason to start such a child network.